i. Technical Field
This invention relates to rear wheel suspensions for bicycles.
ii. Background Art
Conventionally, a bicycle is provided with a frame that is rigid or is flexible only to the extent that particular frame members are flexible. A conventional frame is made of welded generally tubular members, including a generally upright seat tube that supports the seat at its upper end and abuts the bottom bracket, in which the crank rotates, at its lower end; a generally horizontal top tube that is welded at its rear end near the top of the seat tube and at its front end to a head tube, in which the front fork, constituting the front wheel suspension, rotates for steering; a down tube that is welded at its upper front end to the head tube and at its lower rear end to the bottom bracket; and a rear wheel suspension. The seat tube, top tube, and down tube form the rigid generally triangular configuration of the main frame, with the bottom bracket carried at the lower apex. A conventional rigid rear suspension is made up of a pair of generally horizontal chain stays that are welded at their front ends to the bottom bracket and are adapted to receive the rear wheel axle near their rear ends; and a pair of seat stays that are welded at their upper ends near the top of the seat tube and are welded near the ends of the chain stay arms at their lower ends. The seat tube, chain stays, and seat stays form a rigid generally triangular configuration, with the rear wheel carried at the rear apex.
Such a conventional rigid arrangement can be suitable for road or track use, where the riding surface is generally free of irregularities, but is unsatisfactory for comfort and optimum performance off-road. As cyclists have demanded higher performance in off-road bicycles, variously referred to as "mountain bikes", "trail bikes", or "cross-country bikes", bicycle makers have recognized the desirability of providing a pivoting spring suspension for at least the rear wheel.
Pivoting "swingarm" spring suspensions have long been used for rear wheel suspension on motorcycles. In a simplest configuration, the chain stays are replaced by a pair of arms that are pivotally attached at their front ends to the frame at a pivot point near the bottom bracket. The rear ends of the swing arms, carrying the rear wheel, move upward and downward in arcs parallel to the plane described by the rear wheel and centered on the swing arm pivot axis. The range of movement of the swing arms is limited, in a simple configuration, by means such as a spring or a hydraulic shock absorber, affixed between the swing arm and a member of the rigid frame.
Motorcycle frames and swing arms are typically made very heavy, to provide sufficient strength and rigidity to prevent lateral and torsional flex. Simple swingarm suspensions have been adapted for use on bicycles in various configurations. In Kamler et al. U.S. Pat. No. 5,000,470, for example, a pair of swing arms are pivotally attached to the main frame at the bottom bracket, and the range of their upward and downward movement is limited by a pair of shock absorbers that are pivotally attached at their lower ends to pivot points on the swing arms above the rear wheel axle and at their upper ends to a pivot point on the seat tube.
When force is applied from the motor or the pedals through the chain to the rear wheel sprocket, one force component results in forward rotation of the wheel, and another force component tends to urge the wheel axle upward. As a result, in simple swingarm rear suspensions such as are described above, the swing arms pivot sharply upward when a surge of power is supplied to the rear wheel, and pivot downward again when the power is backed off. When a mountain bike, whether motor- or pedal-driven, is worked over rough terrain, this "jacking" can reduce the effective contact of the rear wheel with the riding surface and can severely compromise the rider's control over the bike.
One proposed remedy for the jacking problem in a motorcycle having a simple swingarm suspension is proposed in McGonegle U.S. Pat. No. 4,039,200. McGonegle describes positioning the swing arm pivot axis so that it intersects the tension run of the drive chain, and preferably so that it passes through the tangent point of the drive chain tension run with the power transfer sprocket or the motor drive sprocket.
Jacking further reduces performance because the upward movement of the rear wheel results in a dissipation of power that might otherwise go to forward propulsion of the bike. Such losses may be of little consequence for a motorcycle, but are intolerable to a bicyclist seeking peak performance.
Where caliper brakes are employed on a bicycle, in a simple swingarm suspension the caliper is attached to the swing arms. As a result, braking forces are translated to the swing arms, interfering the response of the suspension to the riding surface when the brakes are applied.
Lawwill U.S. Pat. Nos. 4,789,174 and 5,121,937 propose more complex pivoting rear wheel suspensions, in which a pair of swing arms is pivotally attached about or at the bottom bracket; the rear ends of the swing arms are pivotally attached to the lower end of a pair of rear hub plates, which carry the rear wheel; the upper ends of the rear hub plates are pivotally attached to the rear ends of a pair of control arms; and the front ends of the control arms are attached to a pivot point on the seat tube. In Lawwill '937 the swing arms and the control arms are dimensioned such that the pivot axis at the rear ends of the swing arms is slightly closer to the seat tube than is the pivot axis at the rear ends of the control arms; this careful maintenance of position is said to maintain the suspension in careful balance during pedaling, while at the same time permitting absorption irregularities in the riding surface. In Lawwill '174 power losses are said to be neutralized by virtue of a trapezoid formed by the location of the pivot axes together with the position of the rear wheel axle. Particularly, the point where the control arms pivot on the seat tube is substantially above the rear wheel axle, while the point where the swing arms pivot on the bottom bracket is only slightly below the rear wheel axle. This configuration of the swing arms, hub plates, and control arms is said to balance the forces applied to the rear wheel by the chain during pedaling so as to prevent energy absorbing movement of the rear suspension.